The present disclosure relates to sulfonated telechelic polycarbonates and to methods of producing the same. For example, the disclosure relates, in certain embodiments, to the melt synthesis of sulfonated telechelic polycarbonates and to the compositions produced by such a process.
Polycarbonates are synthetic thermoplastic resins derived from bisphenols and phosgene, or their derivatives. They are linear polyesters of carbonic acid and can be formed from dihydroxy compounds and carbonate diesters, or by ester interchange. Their desired properties include clarity or transparency (i.e. 90% light transmission or more), high impact strength, heat resistance, weather and ozone resistance, good ductility, being combustible but self-extinguishing, good electrical resistance, noncorrosive, nontoxic, etc.
Polycarbonates can be manufactured by processes such as melt polymerization, i.e. melt synthesis. Generally, in the melt polymerization process, polycarbonates may be prepared by co-reacting, in a molten state, dihydroxy compound(s) and a diaryl carbonate ester in the presence of a transesterification catalyst in a Banbury® mixer, twin screw extruder, or other batch stirred reactor designed to handle highly viscous materials, to form a uniform dispersion. Volatile monohydric phenol is removed from the molten reactants by distillation and the polycarbonate polymer is isolated as a molten residue. Melt processes are generally carried out in a series of stirred tank reactors. The reaction can be carried out by either a batch mode or a continuous mode. The apparatus in which the reaction is carried out can be any suitable tank, tube, or column. Continuous processes usually involve the use of one or more continuous-stirred tank reactors (CSTRs) and one or more finishing reactors.
The presence of low concentrations of covalently bonded ionic substituents in organic polymers is well known to produce a consistent effect on their physical and rheological properties. Indeed, ionomers (polymers containing less than 10 mole percent of ionic groups) have been shown to exhibit considerably higher moduli and higher glass transition temperatures compared to those of their non-ionic analogues. Improvements in mechanical and thermal performance are generally attributed to the formation of ionic aggregates, which act as thermo-reversible cross-links and effectively retard the translational mobility of polymeric chains. The thermo-reversible nature of ionic aggregation may address many other disadvantages associated with covalently bonded high molecular weight polymers, such as poor melt processability, high melt viscosity, and low thermal stability at typical processing conditions such as high shear rate and temperature.
It is also reported in the literature that ionic interactions alter the crystallization kinetics and resulting morphology, decreasing the level of crystallinity. Telechelic ionomers (i.e. having only functionalized end groups) provide electrostatic interactions without a deleterious effect on the symmetry of the repeating unit. Moreover, the ionic aggregation will occur only at the end of the chain, giving rise to an electrostatic chain extension while random ionomers give rise to a gel-like or cross linked aggregation. For this reason, lower melt viscosities and higher molecular weights should be more easily obtained for telechelic ionomers compared to random ionomers.
U.S. Pat. No. 5,644,017 reported the preparation of telechelic polycarbonates by melt and interfacial methods. It claimed that polycarbonate ionomers presented a strong non-Newtonian melt rheology behavior along with increased solvent and flame resistance.
The '017 patent reported a melt method for the synthesis of telechelic sulfonated polycarbonates by a one-pot reaction of the phenyl ester of sulfobenzoic acid sodium salt (SBENa), bisphenol-A (BPA), and diphenyl carbonate (DPC). However, this method gave rise to a consistently high amount of degradation products. Furthermore, the material obtained was completely insoluble in dichloromethane. The dark yellow product was not soluble in any common organic solvents, nor in strong solvents such as hexafluoroisopropanol or trifluoroacetic acid, and therefore could not be characterized by GPC or NMR. This insolubility has been ascribed to crosslinking due to the formation of Fries rearrangement by-products. It may be due to the high catalyst content (25 ppm of lithium hydroxide) and/or the temperature program used during polymerization. The '017 patent also claimed two glass transition temperatures (at 148° C. and at 217° C.). This fact suggests the presence of two separable components: one with sulfonated end groups, and one without.
The '017 patent also reported solution methods for the preparation of telechelic sulfonated polycarbonates, via 3- or 4-chlorosulfonyl benzoic acid. Example 2 reported a Tg of 165° C. for the 4-isomer, but no Tg was reported in Example 3 for the 3-isomer. Both polymers had very low molecular weights; the 4-isomer had a Mw of 21,210 or a degree of polymerization (DP) of 44, while the 3-isomer had a much lower Mw (since 20% of the sulfonated end groups were incorporated) and a theoretical DP of only 8. Polycarbonates having a Mw of less than 30,000 are usually not useful because they lack the required mechanical properties. The polycarbonate of Example 3 also contained sulfonated groups as integral parts of the polymer backbone (i.e. not pendant from the chain). However, this type of mixed carbonic-sulfonic anhydride linkage is very thermally unstable and would ultimately cause the polycarbonate to fragment into several chains of lower molecular weight wherever such an anhydride linkage occurred, especially during thermal processing. As such, any polycarbonate with anhydride functionality would not be very useful.
It would be desirable to provide telechelic sulfonated polycarbonates having low crosslinking and high transparency.